I should know better than to ask my Twitter followers to ask science questions, but this one entertained me:
Why do humans have butt cheeks and (almost?) no other animals do? Is it about walking upright?
So, basically yes. At least, that’s part of it. Human butts are made up of muscle and fat, and those thick muscles are needed for maintaining an upright posture. The gluteus maximus is the largest of the muscles in that area, and that helps us walk and stand upright in a stable way.
However, humans also have a lot of fat, and that adds up to a lot of the variation in butt size between humans. We carry a lot more fat than most other primate species, and it’s not clear why. I’ve read some speculations over the last few days — that it’s a good fat storage place for a nomadic, hunter-gatherer lifestyle in the lean times, like the way the fat-tailed dwarf lemur stores fat in its tail; that it’s good to sustain women during breastfeeding and while children are small and can’t fend for themselves (meaning their mothers will move slower and have to do more work to get food).
There’s also speculation that it does indeed (as someone else replied to speculate) provide some protection if you fall on your butt. More likely, perhaps, is the observation that it provides a comfortable way to sit when your body is no longer built for squatting. I also wonder if it is a sexual display thing… But it sounds like it’s mostly speculation.
Short of going into Just So stories, I don’t think there’s a solid answer here, but the tendency to big butts probably started with bipedalism.